The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from shrinking the semiconductor process node (e.g., shrink the process node towards the sub-20 nm node). As semiconductor devices are scaled down, new techniques are needed to maintain the electronic components' performance from one generation to the next. For example, low gate-to-drain capacitance and high breakdown voltage of transistors are desirable for high power applications.
As semiconductor technologies evolve, metal oxide semiconductor field effect transistors (MOSFET) have been widely used in today's integrated circuits. MOSFETs are voltage controlled devices. When a control voltage is applied to the gate of a MOSFET and the control voltage is greater than the threshold of the MOSFET, a conductive channel is established between the drain and the source of the MOSFET. As a result, a current flows between the drain and the source of the MOSFET. On the other hand, when the control voltage is less than the threshold of the MOSFET, the MOSFET is turned off accordingly.
MOSFETs may include two major categories. One is n-channel MOSFETs; the other is p-channel MOSFETs. According to the structure difference, MOSFETs can be further divided into three sub-categories, planar MOSFETs, lateral double diffused MOS (LDMOS) FETs and vertical double diffused MOSFETs. In comparison with other MOSFETs, the LDMOS is capable of delivering more current per unit area because its asymmetric structure provides a short channel between the drain and the source of the LDMOS.
In order to further improve the performance of the LDMOS, a dual gate structure may be employed to increase the breakdown voltage of the LDMOS and the gate charge. However, as the semiconductor process node keeps shrinking, there may be no space to accommodate a dual gate structure.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.